(a) Field of the Invention
The present invention relates to a method for calculating an amount of regenerative braking for an environmentally-friendly vehicle, and more particularly, to a method for calculating an amount of regenerative braking for an environmentally-friendly vehicle capable of improving fuel efficiency and drivability by calculating and applying the amount of regenerative braking based on a state of a high voltage component and a power delivery system and an amount of change of a vehicle speed, for example, a charging limit, a shift level, and deceleration.
(b) Description of the Related Art
As well known to those skilled in the art, environmentally-friendly vehicles such as a hybrid vehicle, an electric vehicle, and a fuel cell vehicle maximize fuel efficiency using a regenerative braking technology.
The regenerative braking technology of the environmentally-friendly vehicle is a technology which applies a counter torque to an electric motor by using energy generated during braking to generate electric energy, stores the generated electric energy in a high voltage battery, and reuses the stored electric energy at the time of driving a vehicle.
In the case of the hybrid vehicle, regenerative braking provides about a 30% fuel efficiency improvement as compared with a conventional vehicle and therefore plays an important part in improving fuel efficiency.
As shown in FIG. 1 (RELATED ART), regenerative braking basically performs braking based on a cooperative control of an electric brake system (EBS) 10 which plays a part in hydraulic braking and a hybrid control unit (HCU) 20.
Referring to FIG. 1, the EBS 10 delivers a regenerative braking tolerance (for example, based on a wheel) based on a demand braking amount of a driver (for example, based on a wheel) to the HCU 20, and the HCU 20 delivers an amount of regenerative braking (based on a wheel) to the EBS 10 and/or a motor control unit (MCU) 30 in consideration of the motor 40, the transmission, and the like.
The EBS 10 performs braking based on the hydraulic (friction) braking using a braking force other than the amount of regenerative braking in the demand braking amount.
However, the regenerative braking relies on an estimate as to how much the braking amount by the motor 40 positioned at a front end of the transmission is actually delivered to the wheel, and therefore the amount of regenerative braking may not be accurately calculated during shifting.
To prevent the above problem from occurring, there is a need to calculate the amount of regenerative braking during shifting based on the amount of regenerative braking before and after the shift. In particular, in order to secure a braking effect, it is important to accurately calculate the braking force of the motor 40 which actually delivers the amount of regenerative braking to the wheel. For this purpose, there is a need to develop logic for the amount of regenerative braking considering a charging limit, a shift level, and a deceleration.
In order to develop the logic for the amount of regenerative braking considering the charging limit, the shift level, and the deceleration, there is a need to consider the following problems of the background and related art.
First, in the motor and the battery which are used for the regenerative braking, charging and discharging may be limited due to various environmental conditions. For example, in the case of the battery, the charging may be limited depending on conditions such as a state of charge (SOC) being too high or low, the temperature being excessively high or low, the battery having a problem, and the like. In the case of the motor, the charging may be limited depending on conditions such as the temperature of the motor being too high, the speed of the motor being too fast, the motor having a problem, and the like.
For example, during the winter season, the charging by the battery may be limited before the battery is preheated, such that the charging of the motor may be limited and the regenerative braking may be limited. In this case, characteristics of the motor are changed and thus the regenerative braking may also be affected.
FIG. 2 (RELATED ART) is a graph illustrating the characteristics of the motor when the charging by the motor is limited and when the charging by the motor is not limited.
When the charging by the motor is not limited, the motor has steady power and steady torque characteristics in a range as shown by a bidirectional solid arrow, but when the charging by the motor is limited, the motor has the steady power and torque characteristics in a range as shown by a bidirectional dotted arrow. In FIG. 2, an area shown by the dotted line is a section in which the torque characteristics of the motor are changed.
As described above, a speed at a point where the torque characteristics of the motor are changed is referred to as a base speed (base Rpm). Consequently, the change in the characteristics of the motor affects the regenerative braking. In particular, when the regenerative braking developed when the charging is not limited is applied to the case in which the charging is limited, the braking effect may be changed.
According to the related art, since the amount of motor braking is calculated without reflecting the charging limit of the motor as described above, the regenerative braking is output too much or output too little at the time of the occurrence of charging limit and thus the hydraulic braking is activated, such that the actual total braking amount may be changed. In this case, a problem of feeling a pushing effect or a sudden effect may be caused.
Second, in the case of a hybrid vehicle equipped with a stepped transmission, a shift is performed during the regenerative braking and the amount of regenerative braking is calculated during the shift, but the logic for the regenerative braking considering the fact that the vehicle has different characteristics depending on a shift stage, that is, the shift level has not yet been applied.
In particular, before and after the shift, a change in a transmission input speed is different depending on an inter-stage ratio (difference between respective stages), but according to the related art, the logic depending on the current shift level is not divided and therefore an error may occur at the time of determining a steady torque area and a steady power area. Therefore, the error occurs in calculating the amount of regenerative braking, which may appear as a change in deceleration effect.
Third, when the vehicle encounters an uphill or a downhill during driving as well as when many people board the vehicle or lots of luggage is loaded in the vehicle, a vehicle load is changed. In this case, even though a driver steps on a brake with equal force, the deceleration of a vehicle is changed. Therefore, there is a need to develop a calculation logic for the amount of regenerative braking considering the deceleration of a vehicle
For example, at the time of the change in deceleration (uphill, downhill, and the like), the motor speed variations before and after the shift may be different from a flat area. The change in deceleration occurs because in the case of the downhill, a vehicle speed reduction is small at the same shift time and a difference in a rotation speed (rpm) is small.
However, since the change in deceleration as described above is not reflected to the related art, the amount of regenerative braking is wrongly calculated and thus the change in deceleration effect may occur. In particular, the difference in the transmission input speed occurs before and after the shift at the time of the change in the vehicle load, but in the related art, the difference is not reflected to a calculation condition and map data of the amount of regenerative braking.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.